NGC 2516 Diamond Cluster
NGC 2516 the Diamond Cluster
This is an excellent illustration of this delightful cluster, and also an example of how to process the photo. ASV member Russell Cockman shows how to process the image to retain the star colours.
Anyone with a modest telescope, a tracking equatorial mount and a camera can have a go at this, it just takes care and practice.
The cluster is in the constellation of Carina (not far from the Southern Cross). it's about the size of the Moon and contains 80-100 blue-white stars, some red giants (which are easily seen in the photo) and a handful of double stars.
Russell took the photo using a monochrome camera behind red, green and blue filters. For each filter, he took ten 60 second exposures, and then combined them to make a "stack". Stacking (using free software) enables a computer to ignore pixels that are much brighter or darker than the majority of pixels.
The result of each stack is a single photo that has much less noise than any one of the single exposures. Less noise means Russell could process the photos without losing the dim image in the noise.
He then recombined the three stacks (red, green and blue) in Photoshop to make a full-colour image.
But that's not all. In the photo, the background is a beautiful black, and you can clearly see dim stars, but the bright stars still have lots of colour. This is a great example of "stretching". Russell has processed the photo to amplify the darkest pixels (which are typically dim stars or areas of nebulosity) without amplifying the brightest pixels (bright stars).
It also helped that Russell slightly underexposed the original thirty photos, preventing any of the pixels on the bright stars from being fully exposed, or "clipped". We describe fully or overexposed stars as "burnt out" as they appear bloated, white, and generally yucky.
A great job, Russell!